Previous solutions for controlling mercury emission in combustion gases have only focused on mercury reduction without consideration of the cost of activated carbon. However, a power plant operator can save a significant amount of money by using only as much of the activated carbon materials as needed to meet the operational goals.
Emissions of hazardous pollutants, such as mercury, from power plants must be controlled due to environmental concerns. Coals contain mercury which is released into the atmosphere as elemental and oxidized mercury along with the flue gases resulting from combustion. Unless the mercury is removed from the coal prior to its combustion, it becomes extremely difficult to remove mercury effectively and economically once it becomes a part of the flue gas. One technology for controlling mercury emission is to inject a suitable sorbent into the combustion gases. Powdered activated carbon and various other sorbents are capable of removing mercury by sorption.
Coals contain many impurities including ash, sulfur, mercury, arsenic, selenium, beryllium, boron, etc. When coal is burned in a furnace, it is converted to carbon dioxide and water producing heat. The ash remains behind as a residue while the majority of other impurities, including sulfur, mercury, and arsenic leave with the combustion gases.
Depending upon the firing practices utilized, the ash is removed as bottom ash or as a combination of bottom and fly ash. The fly ash is that portion of the ash that becomes entrained in the combustion gases and moves around with them into the various parts of the boiler or combustion systems. Since the ash is entrained with the combustion gases, it is removed from the combustion gases before the gases are discharged into the atmosphere through chimneys or stacks. The separation of the entrained ash from the combustion gases is accomplished by utilizing particulate control devices such as electrostatic precipitators and baghouses.
Although mercury emissions from power plants are very small in comparison to ash, sulfur oxides, and nitrogen oxides, mercury emissions are targeted for control due to mercury's tendency to bio-accumulate and its potency as a neurotoxin.
Mercury is emitted from the stacks with the combustion gases in the form of elemental and oxidized mercury. Various methods are currently being employed to control the emission of mercury from the stack gases. Most of the processes require injection of a mercury specific sorbent into the combustion gas stream. The sorbent is injected prior to the particulate control device so that the sorbent containing the adsorbed mercury is removed by the particulate control device together with the fly ash.
Powdered activated carbon is one of the most effective sorbents for mercury removal. The powder activated carbon is blown in by compressed air into the combustion gases upstream of the particulate control device at gas temperatures between 250° F. and 800° F. In the case of cold side electrostatic precipitators and baghouses the temperatures range between 250° F. and 400° F. The hot side electrostatic precipitators operate around 800° F. The powdered carbon works best when the gas temperatures are low.